Ecological and Economic Entomology

A Global Synthesis

Brian Freeman

• Publisher: CAB International
• Year: 2020
• ISBN:
  9781789241204
• Topic: Sciences biologiques
• Subtopic: Entomologie

Ecological and Economic Entomology is a comprehensive advanced text covering all aspects of the role of insects in natural ecosystems and their impacts on human activity. The book is divided into two sections. The first section begins with an outline of the structure, classification and importance of insects, followed by the geographical aspects of plant distribution and the complex defences plants marshal against herbivorous insects. Insect pests affecting plant roots, stem, leaf, and reproductive systems are covered in a comprehensive review. This section also covers insects that are important in medical and veterinary science, paying particular attention to those that transmit pathogens. The section concludes with the beneficial aspects of insects, especially their use in biological control, but also as soil formers and their importance in forensic science. Autecology (or single-species ecology) and its application to pest management is the focus of the second section of the book. Firstly, some general aspects of autecology are examined, including species abundance, competition and speciation, and relevant genetics. The classic general theories of insect population dynamics are reviewed, followed by chapters on life tables, time series analysis and mathematical models in insect populations. The final chapter reviews the application of autecology to the insect pests of forests, farms and orchards and to the control of insect vectors of diseases of humans and livestock. Particular attention is paid to environmentally friendly methods of pest management and the application of Integrated Pest Management (IPM) techniques. This volume is essential reading for professional entomologists and advanced students of agricultural, medical and veterinary entomology, insect ecology and conservation.

1 General Introduction

If the karate-ka (student) shall walk the true path, first he will cast aside all preference.

Tatsuo Shimabuku, Grand Master of Isshin-ryu Karate

1.1 The Importance of Insects

Because of their great numbers and diversity, insects have a considerable impact on human life and industry, particularly away from cities and in the tropics. On the positive side they form a large and irreplaceable part of the ecosystem, especially as pollinators of fruit and vegetable crops and, of course, many wild plants (Section 8.2.1). They also have a place in soil formation (Section 8.2.4) and are being used increasingly in ‘greener’ methods of pest control. Biological control using insects as predators and parasites of pest insects has been developed in the West for over a century, and much longer in China. More recently integrated pest management (IPM) and conservation biological control (CBC) are being deployed to better effect.

Entomology has played a major role in the development of ecology and other branches of biology such as genetics, physiology and behaviour. This is not only because insects form the major part of the terrestrial fauna, but also because they offer a convenient method of study. Their relatively small size leads to easy handling and their abundance facilitates sampling and, in turn, a numerical analysis of the results. Their main disadvantage is that there are too many of them. So many orders, families and species exist that learning about their immense diversity takes a long time and not inconsiderable effort. Also, many people are entomophobic: they just do not like insects and proceed in ignorance to belittle their far-reaching effects on people and the environment. After you read this book you will not be among them.

But of course, some insects have a negative side, and in a few this side is considerable. Before harvest they, together with weeds and pathogens, destroy ~30% of the plants we grow for food and materials. Insects transmit some of these pathogens. While weeds can often reduce pest attack, they can also harbour the pest’s enemies or provide alternative resources for the pest itself. Then in storage, insects, mites, rodents and fungi cause a further 30% loss. Apart from such biotic damage, severe physical conditions such as drought, storms and flooding cause additional losses. For example, under ideal field conditions new wheat varieties (e.g. Agnote and Humber) would give yields of ~16 tonnes/ha, but produce typically about half this under good husbandry. Pre-harvest destruction due only to insects is 10–13% (Pimentel et al., 1984; Thacker, 2002). Losses are probably higher in the Developing World. Hill (1997) and Boyer et al. (2012) estimate 30–40% of total crop losses globally. Polis (1999) states ‘Worldwide, ... insects take about as much crop production as is used by humans’, although other data (Reynolds, 2012a) suggest that this estimate is too high. But without crop protection, chemicals included, losses could be 30% higher than they are with it (Oerke, 2006). Over the years there has been a shift of expenditure – insects now cost us large amounts for crop protection, hopefully to lessen their effect. The annual bill for agricultural insecticides is now approximately US$7 billion in North America, and more than US$20 billion worldwide. Even so, their effectiveness is variable. Naturally, these estimates are but a fraction of the total because the costs of application and wasted time should also be put on the account.

This situation has promoted a huge research effort, but since agriculture in its widest sense is the world’s biggest productive industry, and since research scientists are, in the main, dedicated people working for low salaries, generally this has been cost effective. A Premiership footballer gets twice as much in a week as the average scientist gets in a year. Hopefully, future generations will wonder at this past stupidity. One of the reasons why improving pest control is critical, is that although new crop varieties have greatly increased production (Oerke, 2006), traditional crops had more innate resistance to insect damage. Traditional rice in Asia had losses of only a few per cent, while modern varieties can have losses around 25%. Even so, they produce five times as much grain per hectare and that is the bottom line; a basic principle.

The widespread use of insecticides, fungicides and weed killers has brought environmental pollution in its train. Conversely, in traditional African agriculture little is spent on chemicals, but losses from pests are very apparent. Environmental degradation in Africa is due largely to deforestation; in Europe that has happened already. Only if we can learn to manipulate our environment and agro-ecosystems in particular in permanent ways, with low recurrent expenditure, would we have defeated enemy insects. This has been achieved in a few cases. But ‘permanent’ is euphemistic when applied to biological systems: the playing field is uneven and the rules partly known. While limits exist (Arnold, 1992), evolution works constantly (Haldane, 1954; Trivers, 1985), adjusting the positions of all the living players.

Progress in our favour has continued for several decades, but an idyllic final solution is still far off. Old organic insecticides such as DDT were applied at an average rate of 1 kg/ha; now alpha-cypermethrin is applied at around 10 g/ha. These developments come not only within the ambits of organic chemistry, ecology, largely applied ecological entomology and population dynamics (Berryman, 1991b), but also those of economics and management. And any system used to combat insects must dovetail into the control of other pests, agricultural practice in general and into economics. The nearest to the ideal solution we have come are biological control and IPM, but these methods do not always work and there may still be an environmental backlash (Pimentel et al., 1984). Related techniques of landscape management and CBC are still being developed. But ignorance, lack of aesthetic awareness and avarice continue to be important social factors.

Insects attack not only our field, forest and orchard crops, but also our domesticated animals and ourselves, making it more difficult for us to practise agriculture. There are direct and indirect effects. Both farmers and livestock may suffer insect bites, become victims to their juvenile stages or get infected with insect-borne diseases such as malaria and dengue. As we will see, only a few insect species are pests; most are beneficial. The number of important pests is around 0.1% of the vast number of described insect species (about 1000–2000 species in total). With time, some species gain and others recede in their importance. This is because new crops are developed and grown and new forms of husbandry are devised, but also because agro-chemicals are misused. Insects also migrate and/or change under climatic and evolutionary influences, while management is rarely fully informed.

1.2 Insect Size

In the scale of life, insects are of small to medium size. Mymarid wasps that parasitize insect eggs weigh <1 mg, while at the other end of the scale Goliath beetles may attain 40 g, a range >40,000 mg. Hamilton (1996, p. 386) provides an even greater range: Titanus giganteus from palm logs in Brazil are a million times bigger than ptilinid beetles (Ptinella). Their size relative to other terrestrial animals, their size within the range for insects and even their own species have basic consequences for their physiology and ecology (Price, P.W., 1997). We hear fables of fleas scaled to the weight of a man jumping over famous public buildings. This nonsense is the result of a naïve linear scaling of both weight and power. But weight is a cubic function of body length, whereas muscular power is only a squared function. Doubling in weight produces much less than doubling in power. Small size also allows insects to exploit the great physical variation in their environments (Section 10.1.1). Another critical effect is that the smaller the organism, the greater its surface area (a squared function) relative to its weight. Big animals are in the grip of gravity, little ones are more affected by surface effects. Small insects trapped in a film of water may be unable to free themselves. These relationships give us a glimpse into the foreign physical environment of the insect, for example, how a fly can land upside down on a ceiling.

The world is relatively a much larger place for a small animal than it is for a large one (Hutchinson, 1959; Morse et al., 1985). To continue with insect size discussion, our mymarid wasp develops within the minute confines of a moth’s egg, but the larva of the big moth Manduca sexta may have to eat an entire tomato plant for development. Many phytophagous insects live inside their plant food (endophytic). Some cereal beetles can live out their juvenile lives within single grains. Indeed, the rather low metabolic rate of larval forms allows them to survive in cryptic places. Here oxygen is deficient, but they avoid desiccation and predation. Several moth caterpillars, like the teak defoliator (Section 5.2.1.4(k)), roll up leaves and hide therein, and several species pupate in them or use a small chink in the bark of a tree. The small size of insects in general means that their environment, and especially their hygrothermal environment, is very patchy.

Again because of the physics of scaling, flight is an efficient mode of locomotion for insects (Ellington, 1991; Harrison and Roberts, 2000); we need think only of the prodigious distances covered by migrating butterflies like Vanessa cardui (Section 12.3.3.2). While the rate of energy consumption in flight is much greater than that of crawling, the total energetic cost/km is far less (Kammer and Heinrich, 1978). This is partly due to the physics of flight, but also includes wind assistance and avoidance of obstructions on the ground. This is critical for small insects due to the effect of size on the fractal nature of their world, they can take only small steps on the ground (Section 12.2.6). The evolution of mobility in most adult insects alters their spatial ecology relative to that of their juveniles and to all other terrestrial invertebrates (Section 12.3.4.4(c)). It allows them to migrate, seek resources and control landing, and so are able to exploit fully the three-dimensional nature of their habitat (Roff, 1990; Marden, 2000), attributes extending back more than 350 million years ago (Ma). Flight has continued to evolve over this immense period, allowing longer migration and increasingly sophisticated resource seeking (Section 10.2.4.1). Vagrant insects may migrate >500 km in a single generation. The inception of flight brought about a refinement in risk/reward dynamics (q.v.; means which see, refer to Glossary throughout text) and mitigates the changing heterogeneity of the environment. While parasitic worms produce huge numbers of eggs and larvae that seek new hosts, these are earth bound. Aphids and fruit flies fly aloft, greatly extending their area of environmental scanning.

But to fly an insect requires >12% of its body mass in flight muscle. Since muscle is metabolically expensive to build and maintain, there is often a trade-off between flight and reproductive capability (Zera and Denno, 1997; Marden, 2000). Insect size affects both mobility and reproduction. Insect flight muscle is of two types: the synchronous or neurogenic type in which each neural stimulus produces a single contraction; and the asynchronous or myogenic type in which it produces multiple contractions. Unsurprisingly, wing-beat frequency, wing loading, mass-specific power output and fuel consumption are usually greater in the latter. While insect flight mechanisms have similarities to those of birds, their wings have no intrinsic muscles so changes in wing shape are controlled by structural mechanisms. Indeed, controlled deformability is the essence of insect wing design and function; they are elegantly adapted flexible aerofoils (Wootton, 1992).

Insect shape is another consideration, especially in relation to hygrothermal control. Compact insects like higher Diptera and many beetles have less body surface for a given mass than elongate ones, such as dragonflies, locusts and crane flies. The latter potentially desiccate more easily and some are more affected in flight by wind (Freeman and Adams, 1972). But elongate insects like grasshoppers and asilid flies are better at using differential orientation to gain or lose heat as the situation demands (May, 1979; Morgan and Shelly, 1988; see also Section 10.1.1). When too cool they orientate to cast a big shadow, or when too hot cast a small one, or even take a position in the shade of a grass stem. In some Orthoptera such as Schistocerca and Orphulella, the thorax is flattened ventrally and can be pressed down on warm ground, therefore gaining free heat.

The high metabolic rate of most adult insects contrasts them with nearly all other terrestrial invertebrates, and is a consequence of their efficient system of gaseous exchange: oxygen in and carbon dioxide out. Their unique tracheal system of fine air tubes, assisted in active insects by ventilated air sacs, permits rapid gaseous exchange (Price, P.W., 1997; Dudley, 2000; Harrison and Roberts, 2000). Even simple diffusion of oxygen down tracheae is very much faster than it would be through tissue. Compared to vertebrate blood systems, tracheal systems are light, promoting a higher power : mass ratio. Muscular vibration also assists respiratory exchange, and differences in pressure between the front and back of some flying insects, the Bernoulli entrainment, can pull gases through them. Insect metabolism is unconstrained by a lack of oxygen if they have access to air. Indeed the flight muscle of euglossine bees (Section 8.3.6) is the most active tissue known (Casey et al., 1985; Dudley, 1995). All flight muscles are liberally supplied w...